linux: Enable Landlock support

linux: Enable Landlock support

From the kernel's documentation:

> Landlock is a sandboxing mechanism that enables processes to restrict
> themselves (and their future children) by gradually enforcing
> tailored access control policies. A Landlock security policy is a
> set of access rights (e.g. open a file in read-only, make a
> directory, etc.) tied to a file hierarchy. Such policy can be
> configured and enforced by any processes for themselves using the
> dedicated system calls: landlock_create_ruleset(),
> landlock_add_rule(), and landlock_restrict_self().

There is no harm in enabling this security feature, so applications
supporting Landlock can benefit from it.

Rolled forward from https://patchwork.ipfire.org/project/ipfire/patch/d7ac0caf-5a7c-bcca-6293-16c773523942@ipfire.org/
to submit all kernel-related changes as a single patchset.

Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>